1. Field of the Invention
This invention relates to a suspension system for an automotive vehicle, and, more particularly, to an automotive vehicle suspension system for wheels related to steering.
2. Description of Related Art
While a vehicle turns around a corner, the vehicle body is subjected to a centrifugal force. Because a center of rolling of the vehicle body is generally placed below the center of gravity of the vehicle body at which the centrifugal force acts on the vehicle body, the centrifugal force serves as a rolling moment which takes the distance between the center of gravity and the center of rolling as a lever length. Consequently, a suspension link mechanism between the vehicle body and a wheel driving in an outside path during a turn allows the outside wheel to bump so as to cause the vehicle body to move in close to the outside wheel.
On the other hand, while the vehicle is accelerating, braking or turning, a straightly advancing or propulsive force is applied to the vehicle body at the center of gravity. However, since the instantaneous center of swing of a wheel is generally placed below the center of gravity at which the straightly advancing or propulsive force acts on the vehicle body, the straightly advancing or propulsive force produces a pitching moment proportional to the distance between the center of gravity of the vehicle body and the instantaneous center of swing of the wheel. Consequently, due to the pitching moment, the vehicle is caused to squat during accelerating and to nose dive during braking, and to diagonal roll during a turn.
Reference is made to FIGS. 1 and 2 for the purpose of providing a brief introductory explanation, an instantaneous center of swing P1 of a front wheel 100 (which will be hereafter referred to as a front wheel instantaneous center for simplicity) is defined as a point of intersection of extended lines of upper and lower trailing link levers 101 and 102 of a front wheel suspension link mechanism when viewing the car from the side, and an instantaneous center P2 of a rear wheel 103 (which will be hereafter referred to as a rear wheel instantaneous center) is similarly defined as a point of intersection of extended lines of upper and lower trailing link levers 104 and 105 of a rear wheel suspension link mechanism when viewing the car from the side. The front wheel suspension link mechanism rules its anti-diving property depending upon the ratio of the height (H1) of the front wheel instantaneous center P1 from the ground to the distance, or length, (L1) between the center of the front wheel and the front wheel instantaneous center P1. Further, the front wheel suspension link mechanism rules its anti-squat property depending upon the ratio of the height (H2) of the rear wheel instantaneous center P2 from the ground to the distance, or length, (L2) between the center of the front wheel and the front wheel instantaneous center P2. It is apparent that an increase in the height (H1) of the front wheel instantaneous center P1 and an increase in the height (H2) of the rear wheel instantaneous center P2 provides an enhanced anti-dive property of the vehicle and an enhanced anti-squat property of the vehicle, respectively.
In order to improve the turning property of the vehicle by making a camber, or a camber angle, of a front wheel in the outside path with respect to the ground while turning as small as possible to zero, utilization is made of a suspension geometry that provides a change in camber of a front wheel in the outside path relative to the vehicle body, caused due to bumps of the front wheel, so that the change declines toward negative camber. On the other hand, a caster trail is increased according to degrees of outward bump in order to increase a self-aligning torque, and a caster angle is increased in order to improve driving or handling qualities and enhance the tendency of under steering.
As shown in FIG. 2, when the front wheel bumps as shown by double dotted line, the caster angle, i.e. the inclination of a king pin, and a caster trail Ct are increased and simultaneously, the front wheel instantaneous center is displaced or pulled down to a point P1A from a point P1. This downward displacement of the front wheel instantaneous center P1 of the front wheel 100 deteriorates the properties of anti-diagonal rolling and anti-diving of the vehicle. As apparent from the above description, in order for the vehicle to improve the performance of turning, the suspension geometry has to realize simultaneously the camber property during rolling and the caster property during pitching.
One example of such a front wheel suspension link mechanism, having upper and lower lateral link levers, is known from, for instance, West Germany Laid Open Patent Publication DE3730212. This front wheel suspension link mechanism described in this publication includes an adjusting means to lift up a linking point of the lower lateral link lever to a vehicle body when the vehicle body bumps. With the front wheel suspension link mechanism, since the linking point of the lower lateral link lever is lifted up by means of outside bumps caused during a turn, the depression of the center of rolling of the vehicle body during a turn is declined so as to provide an improved turning performance of the vehicle.
However, because the conventional front wheel suspensions are designed and adapted to increase the caster trail and caster angle of a front wheel according to outside bumps during a turn, a decrease in vertical position of the instantaneous center of a front wheel in the outside path is unavoidably caused according to degrees of turn, namely turning radii or steered angles of a steering wheel, so as to increase a pitching moment which leads to an enhancement of diagonal rolling of the vehicle. For this reason, it may be possible to increase the angle of inclination of the trailing link lever of the front suspension link mechanism so as to place the front wheel instantaneous center in a higher position from the ground. However, if the trailing link lever is inclined at a large angle, it allows the vehicle body to be greatly sensitive to the road surface. This leads to deteriorated ride qualities or comfort and the difficulty of establishing a desirable property of caster. Further, although the stability of vehicle attitude, which depends upon the property of anti-diagonal rolling and anti-diving of the vehicle body, is considerably improved, however, a caster angle is not allowed to be large, so as to provide a strong tendency of over-steering during a turn which in turn leads to a deteriorated stability of driving or handling.
The suspension system described in the above-mentioned publication is structured with an intention of lifting up the center of rolling of the vehicle body and does not control pitching of the vehicle body during bumps. In particular, this suspension system acts in the same or similar manner responding to bumps due to both turning and unevenness of the road surface during straight ahead driving, so as to loose the stability of straight ahead driving due to a sharp change in camber to the ground which is caused by unevenness of the road surface during a turn.